This invention relates to electro-optical ranging systems, particularly systems for taking repetitive and rapid distance measurements to moving target objects, such as for example a rotating log whose surface contour is to be quickly determined prior to performing a processing function thereon so as to enable the adjustment of processing equipment to be set for maximizing the yield of usable wood products from the log.
In the production of wood veneer in the lumber industry it is highly desirable, both economically and ecologically, to maximize the yield of veneer from a given log. To do so, it is necessary to determine, in advance of peeling the log in the lathe, the optimum longitudinal axis for rotation of the log so that it may be mounted in the veneer lathe and rotated about such optimum axis. As explained in an earlier patent of the assignee of the present application, Sohn et al U.S. Pat. No. 3,852,579 entitled "Method and Apparatus for Determining the Surface Configuration of Elongate Objects, Particularly Logs" issued Dec. 3, 1974, the disclosure of which is hereby incorporated in its entirety by reference, one apparatus for finding such optimum axis utilizes a scanning array which is comprised of a plurality of electro-optical transceivers spaced along the length of a log and which, in conjunction with a digital computer, is used to determine the surface contour of the log, prior to its charging into a veneer lathe, as the log is rotated about a longitudinal axis in front of the scanning array. Based on the measurement data provided by the plurality of transceivers, the configuration of the log is determined and the location of the optimum spin axis for peeling is calculated by the digital computer which thereupon controls the charging of the log into the veneer lathe so as to be aligned for rotation about such calculated axis. In such fashion, with use of a mechanized and automatic system of the type described in the aforesaid U.S. Pat. 3,852,579, it is possible for a precise and accurate determination of the optimum axis of rotation for the log to be quickly made in the short interval of time elapsing between when the log is picked up, transported past the scanning array of transceivers while being rotated, and thereafter charged into the veneer lathe to be peeled.
In the transceivers used in the scanning array of the prior art system disclosed in the aforementioned patent, the distance to a point on the surface of a log at a cross-sectional location is determined by measuring the phase shift which occurs between a transmitted and received light beam associated with a particular transceiver unit as the light beam travels to the surface of the log and then is reflected back to the transceiver. This is accomplished by modulating the light beam with a referennce R-F signal and determining the phase shift between the reference signal and the detected envelope of the received light beam. Although the precision of such ranging systems has been improved by using two reference frequencies in heterodyning-type systems such as those disclosed in the aforementioned prior art system, Ruddock U.S. Pat. No. 3,446,971 and Hines et al U.S. Pat. No. 3,778,159, the accuracy of such instruments is difficult to control due to the several threshhold crossings which must be detected in the heterodyning process.
In addition, a problem which is of particular importance in measuring the distance to a moving object such as a rotation log is that the log's reflectivity and thus the intensity of the received light may vary. As the log is rotated past the scanning beam of a given transceiver, changes in its reflectivity result from variations around its perimeter in its natural surface coloration and roughness, and from irregularities such as cracks. This results in a relatively rapid "scintillation" effect whereby variations in reflectivity occur during a given measurement, and in less frequent intensity changes occuring from log-to-log or measurement-to-measurement. Also, variations in the intensity of the received signal of such an optical system will result from the "dirty" environment inherently associated with a log handling apparatus such that particles of wood, dust and other materials interfere with the light beam and collect on the lenses. Such varying intensity makes it difficult to compare the phase of the reference signal to the detected envelope of the received signal because the amplitude of the output signal of R-F phase comparator devices is typically a function of input amplitude as well as relative phase. Thus, rapid, random reflectivity variations, or scintillation, tend to produce errors in a given measurement and less frequent variations tend to produce discrepancies from one measurement to another, thereby decreasing the overall accuracy of the ranging system and in turn the accuracy in determining the optimum spin axis, resulting in wasteful and uneconomical production of veneer. The present invention provides novel features which, among other advantages, enhance the precision and accuracy of an electro-optical ranging system the decrease errors caused by varying reflectivity of the target object.